Overviewing the above considerations on defences, we conclude that many of them fit and reinforce the conclusions drawn at the end of Section 2. The fundamental differences in our expectations of disease pressure across the four groups of social insects that appeared from analysing differences in exposure and transmission (Section 2) are normally not compensated by opposite differences in individual or collective defence (this section). The annual social bees and wasps face higher risks of introducing infections in their colonies when returning from foraging trips because they are more likely to ingest contaminated food away from the colony and have less effective filtering devices to prevent per os infections. Their individual antibiotic defences seem less general and elaborate, and their allogrooming, hygienic behaviour and waste management practices are generally less well developed or less frequent. Although task partitioning probably occurs in all major groups of social insects, physical worker castes that would help in defence against disease are restricted to the perennial ants and termites. In fact, the only factor that is not unambiguously pointing towards a significant advantage in disease defence for the long-lived, perennial societies of ants and termites is intracolonial genetic diversity. Ants are clearly champions in the number of independent transitions to genetically diverse colonies, either because of polygyny or polyandry (Keller and Reeve, 1994; Boomsma and Ratnieks, 1996). They are followed by the social wasps, where polygyny is frequent in the Polistinae (Reeve, 1991) and multiple queen mating occurs in the Vespinae (Foster and Ratnieks, 2001). Next in line are the social bees, where polyandry is almost completely restricted to the honeybees (Palmer and Oldroyd, 2001; Tarpy, 2002) and where polygyny seems less frequent than in the social wasps (Michener, 1974). The termites clearly have the least genetically diverse colonies. The entire group is essentially monogamous, as most documented cases of multiple breeders have been shown to concern offspring reproductives that are on their way to replace parent breeders or to head bud-nests (Thorne, 1985). The only parameter that seems to be correlated with this sequence is the diet: carnivory/omnivory in ants and wasps, followed by a pollen and nectar diet in bees and a decomposer diet in termites (Table 6.1).
We thus expect a number of trends to be apparent in the comparative data. As hosts, we expect ants and termites to be more similar to each other than to the bees and wasps, which should be mutually similar as well. We expect these respective groups of social insect hosts to suffer predominantly from types of parasites and diseases that match their typical nesting and foraging habitats, and we expect these differences to be expressed particularly when grouping parasites and diseases in categories such as the ones proposed in section 2.5.
The next section analyses available comparative data to investigate the extent to which these expectations are supported.
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